Brooks Hays

Sexual conflict among fish can drive the development of bigger
females brains and larger male genitals. Photo by Alex Hofford/European
Pressphoto Agency

New research suggests sexual conflict among fish can encourage both bigger brains and larger genitals.

In lab tests involving eastern
mosquitofish, scientists found males with larger genitals triggered the
development of bigger female brains.

Previous fish studies have shown the
antagonistic relationship among predators and their prey inspires the
evolution of larger brains. More recently, scientists from Sweden and
Australia wanted to find out if the hostile nature of female-male
relationship might yield similar results.

Scientists chose to study the eastern
mosquitofish because of the unseemly nature of their copulation. For
eastern mosquitofish, sex is far from consensual. Males force themselves
upon females. They use a modified anal fin called a gonopodium to
impregnate their victims. Fish with larger gonopodia are better able to
terrorize and penetrate female fish.

In the lab, scientists bred one lineage of
male fish for the trait of enlarged genitals. Another lineage was bred
for small genitals. A third lineage was used as a control. Researchers
released the different groups into tanks of female fish and allowed the
populations to procreate.

After nine generations of evolution,
researchers measured the brain sizes of female fish. The brains of
females subjected to males with longer gonopodia were 6 percent heavier
than the brains of the other two groups.